A subscriber telephone set is generally connected to a telephone exchange by a physical connection constituted by a pair of electric conductors which is reserved for that set. Transmission between the subscriber telephone set and the telephone exchange takes place in the 300-3400 Hz voice band and occupies only a small part of the pass band of the pair. The lower end of this pass band is used for the transmission of a calling signal from the exchange towards the subscriber. This calling signal is a high-voltage low-frequency signal which activates a bell at the subscriber end without any auxiliary energy. The upper end of the pass band of the pair, which is left free by the telephone channel transmitted in the voice band and by the calling signal associated with said channel, can be used for carrier transmission of other telephone channels. This possibility is being used more and more for connecting new subscribers when there is no available pair between them and the telephone exchange. In this type of connection, the telephone channel intended for a new subscriber follows, by means of the carrier connection, a more or less long part of a pair already used for voice band transmission of the telephone channel of another subscriber.
The bell of a telephone subscriber connected to a telephone exchange via a carrier connection cannot be made to ring directly from the exchange. It must draw its power from the subscriber end of the carrier connection. It is generally in the form of an oscillator followed by a power amplifier which draws its power locally. The oscillator operates at the calling frequency; it is blocked or not blocked as a function of the presence or absence of a carrier frequency used in the carrier connection for the frequency transposition of the telephone channel in the exchange-to-subscriber direction.
This disposition has several drawbacks, in particular:
the necessity of using a low frequency output transformer (which is bulky and expensive) in the power amplifier;
the fact that the calling frequency is no longer determined by the telephone exchange but by equipment disposed at the subscriber ends of the carrier connections. This causes a dispersion of the calling frequencies about a rated value, and the dispersion cannot be reduced except by increasing precision or by requiring factory adjustment, which increases the cost of the local oscillator of each equipment. Further, the frequency of the calling signal varies according to the type of use, this also requires adjustment during use.
Preferred embodiments of the present invention avoid the above-mentioned drawbacks.